Is it possible to use multiple octocoupler transistors as switches connected parallel to the coil of the relay? I am worried about slight voltage differences across collector-emitter. Just like the image only multiple parallel octocouplers. edit: I will just use transistor as somebody said in the well known OR configuration
Opto-coupler outputs in parallel is a bad idea unless you include some means for them to share current properly. Just connecting them in parallel doesn't do that.
In this case, the reason seems to be to get higher current capability on the isolated side. That can be achieved with a transistor amplifying the current thru the opto instead of using more optos.
You also seem to be using the same ground on either side of the opto. In that case, lose the opto altogether and replace it with a transistor. Those have much higher gain than optos, so a single transistor can be driven directly from a microcontroller output on one side, and drive the relay directly on the other side. Remember that relays provide isolation too. This circuit would still be isolated between the microcontroller and the switched contacts.
You can parallel the outputs of several opto-couplers to do what you want but beware that output leakage current will build up and, beyond a certain point, the relay may become activated with no active signal going into one of the opto-couplers.